1. Field of the Invention
The invention concerns a munition article which is equipped with a satellite antenna.
2. Discussion of the Prior Art
A munition article of that kind is known from WO 99/02936 A2 in the form of a droppable bomb with sandwich-like or patch-like satellite antenna arranged in the centre of its tail. During the dropping movement into the target area the spherical antenna characteristic thereof maintains contact with navigation satellites which are above the horizon, in order to increase the bomb hit accuracy by means of final phase control, more specifically by comparison between the target point which is predetermined in terms of co-ordinates and the geodetic actual position of the munition article.
Such an antenna configuration is however unsuitable for munition articles in the form for example of artillery shells. For, during the major part of the extended ballistic flight, the antenna directional characteristic which is oriented rearwardly from the tail antenna substantially symmetrically with respect to the longitudinal axis of the projectile would be directed approximately to the horizon, and initially even therebelow, and only slightly thereabove after the apogee. As a result there is a rather low level of probability of being able to receive a plurality of navigation satellites simultaneously with a sufficient level of freedom from interference for rapid and precise trajectory point determination, for the purposes of trajectory correction. Even the incorporation of such a patch antenna into the tip of the projectile would be unsatisfactory because its spherical characteristic which would then be oriented coaxially forwardly would be directed markedly above the horizon only in the very first phase of the ballistic trajectory, in order to be able to detect a plurality of satellites in an appropriate configuration; while the tip, after passing through the apogee, is even directed towards the ground and with such an antenna it would then be possible at best to pick up the very interference-afflicted ground reflections of the signals from navigation satellites, the processing of which sufficiently quickly results in usable items of navigation information, if at all, only by way of considerable signal processing complication and involvement.
Furthermore, in the case of a munition article in the form of an item of artillery munition, in view of the antenna characteristic which is not ideally spherical in practice, the problem of stabilisation rotation arises. For, with an antenna characteristic which is not circular in cross-section, the consequence of the spin which occurs upon launch from a rifled barrel is that the received satellite signals are modulated in dependence on rotation, and that severely adversely affects evaluation of the signals from a plurality of satellites in immediate succession, which signals are to be compared together in rigid phase relationship in themselves for positional determination purposes.
In consideration of those factors the object of the present invention is therefore that of providing a munition article which is fired in the manner of an artillery projectile along a substantially horizontally extended trajectory with an antenna which has an appropriate directional characteristic for the reception of items of satellite navigation information.
In accordance with the invention that object is attained in that an exposed dipole antenna is arranged in the fuse tip of a projectile under a ballistic cap in such a manner whereby the axis of the dipole antenna coincided concentrically with the longitudinal axis of the projectile.
In accordance therewith disposed in the fuse tip of a projectile is a dipole which faces in the direction of flight and which is protectively enclosed by a ballistic cap. The orientation of the exposed dipole axis in the direction of flight guarantees an omnidirectional characteristic with a negligible variation in gain over the rotary angle which is given by the spin of the projectile. The fact that the longitudinal dipole axis concentrically coincides with the longitudinal axis of the projectile provides that the received signal is not modulated in dependence on rotation. The preferred working range of the linearly polarised dipole antenna corresponds to the frequency range of GPS-frequencies. In that case the working range of the dipole antenna is distinguished by the resonance frequency which is established by the zero-passage of the imaginary part of the input impedance in the range of the GPS-frequencies. In order to shorten the dipole length which is required for that purpose to the space available in the projectile, the dipole is equipped with top-loading capacitances which extend the effective length of the dipole by receiving the current. The top-loading capacitances comprise electrically conductive material such as for example metal surfaces or wires. Those conductor portions are arranged rotationally symmetrically about the longitudinal axis of the dipole, whereby the omnidirectional characteristic is guaranteed, and connected to the mutually opposite dipole ends. The conventional dipole radiating device has a half-value width or lobe width of the directional pattern of 78xc2x0. By virtue of the required reduction in the length of the dipole its half-value width moves towards 90xc2x0, which corresponds to the maximum possible half-value width of the Hertzian dipole which acts as the limit case of the shortened antenna. If the xe2x88x9210 dB width of the Hertzian dipole is set as the limit case, that affords a space coverage of 142xc2x0, whereby a large part of the region of space is covered. Narrow regions of space in the direction of flight and in opposite relationship to the direction of flight are not embraced. The conductor portions are inclined at the front end of the dipole towards the axis of the dipole so that in that way they can be fitted into the aerodynamically shaped ballistic cap. At the inside of the ballistic cap which preferably comprises plastic material, it is possible to provide openings and recesses which are of dimensions and in arrangements corresponding to the conductor portions secured to the axis of the dipole. The dipole or the conductor portions are at least partially positively lockingly fitted into that opening or recess.
In a further embodiment the dipole is let into a dielectric with a high dielectric constant, thereby also permitting a reduction in the dipole to the space available in the projectile and guaranteeing a frequency range corresponding to the GPS-frequencies.
Additional developments and further features and advantages of the invention will be apparent from the further claims and from the description hereinafter of a preferred embodiment of the structure according to the invention which is shown diagrammatically and not entirely true to scale and in greatly abstracted form, being restricted to what is essential. This structure affords the advantage that it can be implemented in the fuse tip which is usually removable from an artillery projectile, and therefore can also be subsequently applied to a standard projectile, without having to intervene in the load-bearing structure of the projectile case itself for providing the dipole antenna.